Sintered body and cutting tool

ABSTRACT

A sintered body includes cubic boron nitride grains as hard phase grains, and has a thermal conductivity of not less than 15 W·m −1 ·K −1  and not more than 40 W·m −1 ·K −1 , for cutting a nickel-based heat-resistant alloy formed of crystal grains having a fine grain size represented by a grain size number of more than 5 defined by ASTM standard E112-13. A cutting tool includes this sintered body. Accordingly, the sintered body having both high wear resistance and high fracture resistance, as well as the cutting tool including the sintered body are provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.15/300,115, filed Sep. 28, 2016, which is a 371 of InternationalApplication No. PCT/JP2016/054394, filed on Feb. 16, 2016, which claimsthe benefit of Japanese Application No. 2015-037062, filed on Feb. 26,2015.

TECHNICAL FIELD

The present invention relates to a sintered body for cutting anickel-based heat-resistant alloy and to a cutting tool including thissintered body, and particularly relates to a sintered body for cutting anickel-based heat-resistant alloy formed of crystal grains with a finegrain size, and to a cutting tool including this sintered body.

BACKGROUND ART

A nickel-based heat-resistant alloy is an alloy based on nickel to whichchromium, iron, niobium, molybdenum, and the like are added. Thenickel-based heat-resistant alloy is excellent in high-temperaturecharacteristics such as thermal resistance, corrosion resistance,oxidation resistance, and creep resistance, and suitable for use inapplications requiring thermal resistance, such as aircraft jet engine,automobile engine, and industrial turbine. However, the nickel-basedheat-resistant alloy is a material difficult to cut.

As a cutting tool for cutting such a nickel-based heat-resistant alloy,a cutting tool has been proposed including a sintered body whichcontains cubic boron nitride having the second highest strength afterdiamond and having high wear resistance.

WO00/47537 (PTD 1) for example discloses, as a sintered body to beincluded in the cutting tool as described above, a sintered body withhigh crater resistance and high strength containing 50 vol % to 78 vol %of high pressure phase boron nitride and a balance of a binder phase.Japanese Patent Laying-Open No. 2000-226262 (PTD 2) also discloses ahigh-hardness high-strength sintered body produced by sintering hardgrains which are high-pressure-type boron nitride grains each coveredwith a coating layer, and a binder phase uniting the hard grains.Moreover, Japanese Patent Laying-Open No. 2011-140415 (PTD 3) disclosesa sintered body containing cubic boron nitride, a first compound, and asecond compound, in which the content of the cubic boron nitride is notless than 35 vol % and not more than 93 vol %.

CITATION LIST Patent Document

-   PTD 1: WO00/47537-   PTD 2: Japanese Patent Laying-Open No. 2000-226262-   PTD 3: Japanese Patent Laying-Open No. 2011-140415

SUMMARY OF INVENTION Technical Problem

A problem of respective sintered bodies disclosed in WO00/47537 (PTD 1),Japanese Patent Laying-Open No. 2000-226262 (PTD 2), and Japanese PatentLaying-Open No. 2011-140415 (PTD 3) is that the fracture resistance ofthe sintered bodies is not high while the wear resistance is high whenthe sintered bodies are used for cutting a workpiece. Fracture of thecutting tool is a critical problem when used for cutting parts of anaircraft jet engine, an automobile engine, and the like for which highdimensional accuracy and high surface quality are required. Particularlywhen the cutting tool is used for cutting a nickel-based heat-resistantalloy formed of crystal grains with a fine grain size, specifically agrain size number of more than 5 defined by American Society for Testingand Materials (hereinafter also referred to as ASTM) standard E112-13,there is a problem that a fracture called boundary damage is likely tooccur to a cutting blade of the cutting tool, in addition to normal wearof the flank face of the cutting tool. Therefore, a sintered bodyexhibiting both high wear resistance and high fracture resistance whenused for cutting a workpiece is required.

An object is therefore to provide a sintered body having both high wearresistance and high fracture resistance, as well as a cutting toolincluding this sintered body.

Solution to Problem

A sintered body in an aspect of the present invention is a sintered bodyincluding cubic boron nitride grains as hard phase grains, and having athermal conductivity of not less than 15 W·m⁻¹·K⁻¹ and not more than 40W·m⁻¹·K⁻¹, for cutting a nickel-based heat-resistant alloy formed ofcrystal grains having a fine grain size represented by a grain sizenumber of more than 5 defined by American Society for Testing andMaterials standard E112-13.

A cutting tool in another aspect of the present invention is a cuttingtool including the sintered body as described above.

Advantageous Effects of Invention

According to the foregoing, a sintered body having both high wearresistance and high fracture resistance, as well as a cutting toolincluding this sintered body can be provided.

DESCRIPTION OF EMBODIMENTS Description of Embodiments of the Invention

A sintered body in an embodiment of the present invention is a sinteredbody including cubic boron nitride grains as hard phase grains, andhaving a thermal conductivity of not less than 15 W·m⁻¹·K⁻¹ and not morethan 40 W·m⁻¹·K⁻¹, for cutting a nickel-based heat-resistant alloyformed of crystal grains having a fine grain size represented by a grainsize number of more than 5 defined by American Society for Testing andMaterials (hereinafter also referred to as ASTM) standard E112-13. Thesintered body in the present embodiment has a thermal conductivity ofnot less than 15 W·m⁻¹·K⁻¹ and not more than 40 W·m⁻¹·K⁻¹, and thereforeexhibits both high wear resistance derived from the cubic boron nitridegrains and high fracture resistance when used for cutting a nickel-basedheat-resistant alloy which is formed of crystal grains having a finegrain size represented by a grain size number of more than 5 defined byASTM standard E112-13.

The sintered body in the present embodiment may further include a binderand different-type hard phase grains including at least one selectedfrom the group consisting of silicon nitride, SiAlON, and alumina, asthe hard phase grains other than the cubic boron nitride grains. Thissintered body thus further includes a binder and different-type hardphase grains including at least one selected from the group consistingof silicon nitride, SiAlON, and alumina, as the hard phase grains otherthan the cubic boron nitride grains, to thereby exhibit both high wearresistance and high fracture resistance when used for cutting anickel-based heat-resistant alloy formed of crystal grains having a finegrain size represented by a grain size number of more than 5 defined byASTM standard E112-13.

Regarding the sintered body in the present embodiment, a ratioV_(BN)/V_(H) of a volume V_(BN) of the cubic boron nitride grains to avolume V_(H) of the different-type hard phase grains may be not lessthan 1 and not more than 6. This sintered body thus has a ratioV_(BN)/V_(H) of not less than 1 and not more than 6, as a ratio of avolume V_(BN) of the cubic boron nitride grains to a volume V_(H) of thedifferent-type hard phase grains, to thereby have both high wearresistance and high fracture resistance.

Regarding the sintered body in the present embodiment, the SiAlON mayinclude cubic SiAlON. This sintered body thus includes cubic SiAlONwhich has low reactivity to the metal and higher hardness than those ofα-SiAlON and β-SiAlON, to thereby have higher wear resistance.

The SiAlON may further include at least one of α-SiAlON and β-SiAlON,and a peak intensity ratio Rc of an intensity at an X-ray diffractionmain peak of the cubic SiAlON to a sum of respective intensities atrespective X-ray diffraction main peaks of the α-SiAlON, the β-SiAlON,and the cubic SiAlON may be not less than 20%. This sintered body thusincludes the cubic SiAlON, and at least one of α-SiAlON and β-SiAlON,and has a ratio of 20% or more of the cubic SiAlON to the sum of theα-SiAlON, the β-SiAlON, and the cubic SiAlON, in term of the intensityat a main peak of X-ray diffraction. Accordingly, the sintered body hasboth high wear resistance and high fracture resistance.

Regarding the sintered body in the present embodiment, the binder mayinclude at least one kind of binder selected from the group consistingof at least one kind of element out of titanium, zirconium, aluminum,nickel, and cobalt, nitrides, carbides, oxides, carbonitrides, andborides of the elements, and solid solutions thereof. In this sinteredbody, the binder strongly bonds the different-type hard phase grains andthe cubic boron nitride grains, and increases the fracture toughness ofthe sintered body. The sintered body therefore has higher fractureresistance.

Regarding the sintered body in the present embodiment, a content of thehard phase grains in the sintered body may be not less than 60 vol % andnot more than 90 vol %. This sintered body has well-balanced high wearresistance and high fracture resistance.

Regarding the sintered body in the present embodiment, the sintered bodymay have a Vickers hardness of not less than 22 GPa. This sintered bodythus has a Vickers hardness of not less than 22 GPa, and therefore hashigh wear resistance.

Regarding the sintered body in the present embodiment, the nickel-basedheat-resistant alloy may be Inconel® 718. This sintered body alsoexhibits both high wear resistance and high fracture resistance whenused for cutting Inconel® 718 formed of crystal grains with a fine grainsize represented by a grain size number of more than 5 defined by ASTMstandard E112-13, which is a typical example of the nickel-basedheat-resistant alloy.

A cutting tool in another embodiment of the present invention is acutting tool including the sintered body in the aforementionedembodiment. The cutting tool in the present embodiment includes thesintered body in the aforementioned embodiment, and therefore exhibitsboth high wear resistance and high fracture resistance when used forcutting a nickel-based heat-resistant alloy which is formed of crystalgrains having a fine grain size represented by a grain size number ofmore than 5 defined by ASTM standard E112-13.

DETAILS OF EMBODIMENTS OF THE INVENTION First Embodiment: Sintered Body

{Sintered Body}

A sintered body in an embodiment of the present invention is a sinteredbody including cubic boron nitride grains as hard phase grains, andhaving a thermal conductivity of not less than 15 W·m⁻¹·K⁻¹ and not morethan 40 W·m⁻¹·K⁻¹, for cutting a nickel-based heat-resistant alloyformed of crystal grains having a fine grain size represented by a grainsize number of more than 5 defined by American Society for Testing andMaterials (ASTM) standard E112-13. Crystal grains having a smaller grainsize number are coarser crystal grains. Regarding the nickel-basedheat-resistant alloy to be cut by means of the sintered body in thepresent embodiment, the grain size number of 5 or less corresponds to acrystal grain size of about 50 μm or more. The sintered body in thepresent embodiment has a thermal conductivity of not less than 15W·m⁻¹·K⁻¹ and not more than 40 W·m⁻¹·K⁻¹, and therefore exhibits bothhigh wear resistance and high fracture resistance when used for cuttinga nickel-based heat-resistant alloy which is formed of crystal grainshaving a fine grain size represented by a grain size number of more than5 defined by ASTM standard E112-13.

In order to develop a sintered body exhibiting both high wear resistanceand high fracture resistance when used for cutting a nickel-basedheat-resistant alloy which is formed of crystal grains having a finegrain size represented by a grain size number of more than 5 defined byASTM standard E112-13, the inventors of the present invention initiallyexamined the relation between cutting resistance and damage to a cuttingblade. The cutting resistance is the cutting resistance against thecutting blade of a cutting tool including the sintered body containingcubic boron nitride grains with high wear resistance, when cutting anickel-based heat-resistant alloy. As a result of this, the followingwas found. When a nickel-based heat-resistant alloy was cut, the alloywas cut with a significantly higher cutting resistance as compared withthe cutting resistance when cutting a hardened steel which is also adifficult-to-cut material. Therefore, due to contact with swarf withhigh hardness, a deep boundary damage in a V-shape as seen from theflank face of the tool was generated in the tool. It was also found thatthe boundary damage extending into the cutting blade caused decrease ofthe strength of the cutting edge.

The inventors of the present invention considered that a cause of such aboundary damage was decrease of the temperature of the cutting edgeduring cutting, due to the high thermal conductivity of the cubic boronnitride grains forming the cutting blade.

In the sintered body with a high content of cubic boron nitride grainshaving the second highest thermal conductivity after diamond grains,necking between the cubic boron nitride grains occurs in the sinteredbody to form a three-dimensional mesh structure. Therefore, the thermalconductivity increases through this three-dimensional mesh structure.Particularly in the case where the sintered body includes a metal bindersuch as cobalt (Co) or aluminum (Al), as a binder of the cubic boronnitride grains, the thermal conductivity of the sintered body is furtherincreased by the high thermal conductivity of the metal binder itself,to a thermal conductivity of 70 w·m⁻¹·K⁻¹.

The inventors of the present invention examined the relation between thecutting resistance and the thermal conductivity of the sintered bodyincluding the cubic boron nitride grains forming the cutting blade ofthe cutting tool. As a result, the inventors found that increase of thethermal conductivity of the sintered body caused increase of the cuttingresistance when a Ni-based heat resistant alloy such as Inconel® is cut.When a Ni-based heat-resistant alloy is cut, the temperature at aportion where a workpiece (work) and the cutting edge of the cuttingtool contact each other increases to approximately 700° C., andaccordingly the workpiece at the contact portion is softened. Then, thedeforming stress decreases and accordingly the cutting resistancedecreases. However, when cutting is performed with a cutting tool whichis formed of a sintered body having a high content of cubic boronnitride grains and having a three-dimensional mesh structure of thegrains, and which has high cooling ability, it is considered that thetemperature of the cutting edge during cutting is kept at a lowtemperature, and therefore, the workpiece is not softened and thecutting resistance increases.

As set forth above, the inventors of the present invention examined therelation between the cutting resistance and the thermal conductivity ofthe sintered body forming the cutting blade of the cutting tool andincluding cubic boron nitride grains, and consequently found that ahigher thermal conductivity of the sintered body forming the cuttingblade of the cutting tool caused a higher cutting resistance and agreater damage to the cutting blade.

Further, the inventors of the present invention exhaustively performedcutting of workpieces which were a plurality of nickel-basedheat-resistant alloys different from each other in grain size of crystalgrains, and consequently found that a coarser grain size of the crystalgrains of the nickel-based heat-resistant alloy was accompanied by ahigher cutting resistance during the cutting. In particular, it wasfound that, when a nickel-based heat-resistant alloy was cut that wasformed of crystal grains with a coarse grain size represented by a grainsize number of 5 or less defined by ASTM standard E112-13, the cuttingtool reached the end of the life in a considerably short time due tofracture, before wear increased. In contrast, it was found that, when anickel-based heat-resistant alloy was cut which was formed of crystalgrains with a fine grain size represented by a grain size number of morethan 5 defined by ASTM standard E112-13, and which may be used for heatresistant parts of a turbine disk of an aircraft engine or the likerequired to have creep resistance, the depth of a damage to the cuttingblade of the cutting tool was smaller as compared with the case where anickel-based heat-resistant alloy was cut that was formed of crystalgrains with a coarse grain size represented by a grain size number of 5or less defined by ASTM standard E112-13. However, it was found that theaforementioned boundary damage occurred and normal wear occurred to thetool flank face.

Generally, the material for the cutting tool is often required to havehigh thermal conductivity for the purpose of preventing plasticdeformation (thermal deformation) or thermal cracks of the cutting toolitself. However, the inventors of the present invention found that, inthe case of cutting a nickel-based heat-resistant alloy formed ofcrystal grains with a fine grain size represented by a grain size numberof more than 5 defined by ASTM standard E112-13, increase of the thermalconductivity of the material for the cutting tool is accompanied byincrease of a boundary damage of the cutting edge of the cutting bladeand increase of the cutting resistance, and accordingly the cutting edgeof the cutting blade is likely to fracture. Therefore, contrary to theconventional approach, the inventors tried decreasing the thermalconductivity of the sintered body including cubic boron nitride grains.

As a result of this trial, the inventors found that the grain size ofthe cubic boron nitride powder used as a material for the sintered bodycould be made finer and an inorganic compound such as TiN, TiC, TiAlN,or AlB₂ could be used as a binder to thereby decrease the thermalconductivity of the sintered body. Preferably, the cubic boron nitridepowder has an average grain size of 1.5 μm or less.

Alternatively, silicon nitride, SiAlON, alumina, or the like withcrystal grains having lower thermal conductivity than cubic boronnitride grains was added to the sintered body to thereby obtain athermal conductivity of the sintered body which was an intermediatethermal conductivity between that of the conventional ceramic tool andthat of a cubic boron nitride tool, specifically a thermal conductivityof not less than 15 W·m⁻¹·K⁻¹ and not more than 40 W·m⁻¹·K⁻¹. It wasfound that a sintered body could thus be obtained which exhibited bothhigh wear resistance and high fracture resistance when used for cuttinga nickel-based heat-resistant alloy formed of crystal grains with a finegrain size represented by a grain size number or more than 5 defined byASTM standard E112-13, and accordingly the present invention wascompleted.

In order for the sintered body in the present embodiment to have bothhigh wear resistance and high fracture resistance, the sintered bodyincludes cubic boron nitride grains and still has a thermal conductivityof not less than 15 W·m⁻¹·K⁻¹ and not more than 40 W·m⁻¹·K⁻¹, preferablynot less than 20 W·m⁻¹·K⁻¹ and not more than 40 W·m⁻¹·K⁻¹, and morepreferably not less than 20 W·m⁻¹·K⁻¹ and not more than 35 w·m⁻¹·K⁻¹. Ifthe thermal conductivity of the sintered body is more than 40 W·m⁻¹·K⁻¹,the temperature of the cutting edge of the cutting tool formed of thissintered body may decrease to become less than the softening temperatureof the workpiece, resulting in insufficient suppression of boundarydamage to the cutting edge of the cutting blade. If the thermalconductivity of the sintered body is less than 15 W·m⁻¹·K⁻¹, the cuttingtemperature may become excessively high, resulting in promotion of wearof the cutting tool formed of this sintered body.

The thermal conductivity of the sintered body is determined in thefollowing way. From the sintered body, a sample with a diameter of 18 mmand a thickness of 1 mm is cut as a sample to be used for measuring thethermal conductivity, and a laser-flash-method thermal constantmeasuring apparatus is used to measure the specific heat and the thermaldiffusivity. The thermal conductivity is calculated by multiplying thethermal diffusivity by the specific heat and the density of the sinteredbody.

Preferably, the sintered body in the present embodiment further includesa binder and different-type hard phase grains including at least oneselected from the group consisting of silicon nitride, SiAlON, andalumina, as the hard phase grains other than the cubic boron nitridegrains. This sintered body thus further includes: the different-typehard phase grains which are grains of at least one selected from thegroup consisting of silicon nitride, SiAlON, and alumina; the cubicboron nitride grains; and the binder, to thereby exhibit both high wearresistance and high fracture resistance when used for cutting anickel-based heat-resistant alloy formed of crystal grains having a finegrain size represented by a grain size number of more than 5 defined byASTM standard E112-13.

Regarding the sintered body in the present embodiment, a ratioV_(BN)/V_(H) of a volume V_(BN) of the cubic boron nitride grains to avolume V_(H) of the different-type hard phase grains is preferably notless than 1 and not more than 6. This sintered body thus has a ratioV_(BN)/V_(H) of not less than 1 and not more than 6, as a ratio of thevolume of the cubic boron nitride grains to the volume of thedifferent-type hard phase grains, to thereby have both high wearresistance and high fracture resistance. If the ratio V_(BN)/V_(H) isless than 1, the content of the cubic boron nitride grains having highhardness is relatively low, resulting in decrease of the hardness of thesintered body, which may cause decrease of the wear resistance of acutting tool formed of this sintered body. In contrast, if the ratioV_(BN)/V_(H) is more than 6, the cubic boron nitride grains having highthermal conductivity are excessively present in the sintered body, whichmay make it impossible to have a thermal conductivity of 40 W·m⁻¹·K⁻¹ orless.

Regarding the sintered body in the present embodiment, a predeterminedamount of the different-type hard phase grains in a powder state and apredetermined amount of the cubic boron nitride grains in a powder stateare added and mixed before being sintered. It was confirmed that whenX-ray diffraction was performed before and after sintering, there was nosignificant change in peak intensity ratio between the different-typehard phase grains and the cubic boron nitride grains and the volumeratio between the different-type hard phase grains and the cubic boronnitride grains added in the powder state was substantially maintained asit was in the sintered body. Therefore, X-ray diffraction of thesintered body is performed and a ratio V_(BN)/V_(H) of a volume V_(BN)of the cubic boron nitride grains to a volume V_(H) of thedifferent-type hard phase grains can be calculated from the X-raydiffraction peak intensity ratio between the different-type hard phasegrains and the cubic boron nitride grains. Other than theabove-described X-ray diffraction, a CP (cross section polisher)(manufactured by JEOL Ltd.) or the like may be used to mirror polish asintered-body cross section, observe the cross section with an SEM(scanning electron microscope), examine constituent elements of crystalgrains by means of EDX (energy dispersive X-ray spectrometry), andidentify the different-type hard phase grains and the cubic boronnitride grains, to thereby determine an area ratio therebetween to beregarded as a volume ratio. In this way, the ratio V_(BN)/V_(H) of avolume V_(BN) of the cubic boron nitride grains to a volume V_(H) of thedifferent-type hard phase grains can also be calculated.

Regarding the sintered body in the present embodiment, preferably theSiAlON includes cubic SiAlON. This sintered body thus includes cubicSiAlON which has low reactivity to the metal and higher hardness thanthose of α-SiAlON and β-SiAlON, to thereby have higher wear resistance.

Preferably, the SiAlON further includes at least one of α-SiAlON andβ-SiAlON, and a peak intensity ratio Rc of an intensity at an X-raydiffraction main peak of the cubic SiAlON to a sum of respectiveintensities at respective X-ray diffraction main peaks of the α-SiAlON,the β-SiAlON, and the cubic SiAlON is not less than 20% (the peakintensity ratio is hereinafter also referred to as peak intensity ratioRc of the cubic SiAlON). This sintered body thus includes the cubicSiAlON and at least one of α-SiAlON and β-SiAlON, and the ratio, interms of the intensity at the X-ray diffraction main peak, of the cubicSiAlON to the sum of the α-SiAlON, the β-SiAlON, and the cubic SiAlON isnot less than 20%. Accordingly, the sintered body has both high wearresistance and high fracture resistance.

Peak intensity ratio Rc of the cubic SiAlON is an index corresponding tothe ratio of the cubic SiAlON to the different-type hard phase grains.The peak intensity ratio Rc of the cubic SiAlON may be determined asfollows. The sintered body is surface-ground with a diamond abrasive(hereinafter referred to as #400 diamond abrasive) formed of diamondabrasive grains passing a #400 sieve (a sieve with a mesh size of 38μm). From an X-ray diffraction pattern obtained by measuring the groundsurface by means of characteristic X-ray of Cu-Kα, a peak intensityIc₍₃₁₁₎ of (311) plane which is a main peak of the cubic SiAlON, a peakintensity Iα₍₂₀₁₎ of (201) plane which is a main peak of the α-SiAlON,and a peak intensity Iβ₍₂₀₀₎ of (200) plane which is a main peak ofβ-SiAlON, can be determined. The values of these peak intensities can beused to calculate peak intensity ratio Rc of the cubic SiAlON based onthe following formula (I). If peak intensity ratio Rc of the cubicSiAlON is less than 20%, the hardness of the sintered body decreases,and the wear resistance may decrease.

Rc=Ic ₍₃₁₁₎/(Ic ₍₃₁₁₎ +Iα ₍₂₀₁₎ +Iβ ₍₂₀₀₎₎×100  (I)

Regarding the sintered body in the present embodiment, preferably thebinder includes at least one kind of binder selected from the groupconsisting of at least one kind of element out of titanium (Ti),zirconium (Zr), aluminum (Al), nickel (Ni), and cobalt (Co), nitrides,carbides, oxides, carbonitrides, and borides of the elements, and solidsolutions thereof. In this sintered body, the binder strongly bonds thedifferent-type hard phase grains and the cubic boron nitride grains, andincreases the fracture toughness of the sintered body. The sintered bodytherefore has high fracture resistance.

As this binder, a metal element such as Al, Ni, Co, an intermetalliccompound such as TiAl, or a compound such as TiN, ZrN, TiCN, TiAlN,Ti₂AlN, TiB₂, AlB₂, for example, is suitably used. In the sintered bodyincluding this binder, the different-type hard phase grains and thecubic boron nitride grains are strongly bonded. In addition, in the casewhere the fracture toughness of the binder itself is high, the fracturetoughness of the sintered body is accordingly high, and thus thefracture resistance of the sintered body is high.

Regarding the sintered body in the present embodiment, the content ofthe hard-phase grains in the sintered body is preferably not less than60 vol % and not more than 90 vol % (the content refers to the contentof the cubic boron nitride grains when the cubic boron nitride grainsare included as hard-phase grains, and refers to the total content ofthe different-type hard phase grains and the cubic boron nitride grainswhen the different-type hard phase grains and the cubic boron nitridegrains are included as hard-phase grains; therefore, the content ofhard-phase grains may be defined as the total content of thedifferent-type hard phase grains and the cubic boron nitride grainsregardless of whether the different-type hard phase grains are presentor not, as the content of the different-type hard phase grains may beregarded as 0 vol % when the hard-phase grains do not include thedifferent-type hard phase grains). This sintered body has well-balancedhigh wear resistance and high fracture resistance. If the content ofhard-phase grains (the total content of the different-type hard phasegrains and the cubic boron nitride grains) is less than 60 vol %, thesintered body has a lower hardness, which may result in lower wearresistance. If the content of hard-phase grains (the total content ofthe different-type hard phase grains and the cubic boron nitride grains)is more than 90 vol %, the sintered body has a lower fracture toughness,which may result in lower fracture resistance.

Regarding the sintered body in the present embodiment, a predeterminedamount of the different-type hard phase grains in a powder state, apredetermined amount of the cubic boron nitride grains in a powderstate, and a predetermined amount of the binder in a powder state areadded and mixed before being sintered. It was confirmed that when X-raydiffraction was performed before and after sintering, there was nosignificant change in peak intensity ratio between the different-typehard phase grains, the cubic boron nitride grains, and the binder, andthe volume ratio between the different-type hard phase grains, the cubicboron nitride grains, and the binder added in the powder state wassubstantially maintained as it was in the sintered body. Other than theabove-described X-ray diffraction, a CP or the like may be used tomirror polish a sintered-body cross section, observe the cross sectionwith an SEM, examine constituent elements of crystal grains by means ofEDX, and identify the different-type hard phase grains, the cubic boronnitride grains, and the binder to thereby determine an area ratiotherebetween to be regarded as a volume ratio. In this way as well, thevolume ratio between the different-type hard phase grains, the cubicboron nitride grains, and the binder included in the sintered body canbe determined.

Regarding the sintered body in the present embodiment, the sintered bodyhas a Vickers hardness of preferably not less than 22 GPa, and morepreferably not less than 28 GPa. This sintered body thus has a Vickershardness of not less than 22 GPa, and therefore has high wearresistance. If the Vickers hardness is less than 22 GPa, the wearresistance may be low.

The Vickers hardness of the sintered body in the present embodiment maybe measured as follows. The sintered body embedded in a Bakelite resinis polished for 30 minutes with diamond abrasive grains of 9 μm and for30 minutes with diamond abrasive grains of 3 μm. After this, a Vickershardness tester is used to press a diamond indenter into the polishedsurface of the sintered body with a load of 10 kgf. From the indentationformed by the pressing of the diamond indenter, the Vickers hardnessH_(V10) is determined. Further, the length of a crack extending from theindentation is measured. Based on the IF (Indentation-Fracture) methodunder JIS R 1607: 2010 (Testing methods for fracture toughness of fineceramics at room temperature), the fracture toughness is determined.

Regarding the sintered body in the present embodiment, the nickel-basedheat-resistant alloy is preferably Inconel® 718. This sintered body alsoexhibits high fracture resistance in addition to high wear resistancewhen used for cutting Inconel® 718 formed of crystal grains with a finegrain size represented by a grain size number of more than 5 defined byASTM standard E112-13, which is a typical example of the nickel-basedheat-resistant alloy.

Inconel® 718 is an alloy mainly including 50 to 55 mass % of nickel(Ni), 17 to 21 mass % of chromium (Cr), 4.75 to 5.50 mass % of niobium(Nb), 2.80 to 3.30 mass % of molybdenum (Mo), and about 12 to 24 mass %of iron (Fe), for example. Inconel® 718 is excellent in high-temperaturestrength provided by an Nb compound generated through age-hardening, andused for aircraft jet engine and various high-temperature structuralmembers. Meanwhile, in terms of cutting, Inconel® 718 is adifficult-to-cut material which promotes wear of the cutting tool due tohigh affinity with the tool material, and which is likely to causefracture of the tool due to the large high-temperature strength of theworkpiece.

{Method of Manufacturing Sintered Body}

The method of manufacturing the sintered body in the present embodimentis not particularly limited. In order to efficiently manufacture thesintered body having both high wear resistance and high fractureresistance, the method includes the step of preparing different-typehard phase powder, the step of mixing the different-type hard-phasepowder, cubic boron nitride powder, and binder powder, and the sinteringstep. The method will hereinafter be described in the order of thesteps.

Step of Preparing Different-Type Hard Phase Powder

As the different-type hard phase powder, β-SiAlON powder and c-SiAlONpowder synthesized in the following way may be used, in addition tosilicon nitride powder and alumina powder having an average grain sizeof 5 μm or less.

β-SiAlON represented by a chemical formula: Si_(6-Z)Al_(Z)O_(Z)N_(8-Z)(where z is larger than 0 and not more than 4.2) may be synthesized fromsilica (SiO₂), alumina (Al₂O₃), and carbon (C) as starting materials,using the general carbon reduction nitriding method, in a nitrogenambient at atmospheric pressure.

Powder of β-SiAlON may also be obtained by using a high-temperaturenitriding synthesis method to which applied nitriding reaction of metalsilicon in a nitrogen ambient at atmospheric pressure or more, asrepresented by the following formula (II).

3(2−0.5Z)Si+ZAl+0.5ZSiO₂+(4−0.5Z)N₂

→Si_(6-Z)Al_(Z)O_(Z)N_(8-Z)  (II)

Si powder (with an average grain size of 0.5 to 45 μm and a purity of96% or more, more preferably 99% or more), SiO₂ powder (with an averagegrain size of 0.1 to 20 μm), and Al powder (with an average grain sizeof 1 to 75 μm) are weighed in accordance with a desired value of Z, andthereafter mixed with a ball mill or shaker mixer or the like, tothereby prepare material powder for synthesizing β-SiAlON. At this time,other than the above formula (II), aluminum nitride (AlN) and/or alumina(Al₂O₃) may be combined appropriately as Al components. The temperatureat which β-SiAlON powder is synthesized is preferably 2300 to 2700° C.Moreover, the pressure of nitrogen gas filling a container in whichβ-SiAlON powder is synthesized is preferably 1.5 MPa or more. As asynthesis apparatus which can endure such a gas pressure, a combustionsynthesis apparatus or HIP (hot isostatic pressing) apparatus issuitable. Moreover, commercially available α-SiAlON powder and β-SiAlONmay be used.

Subsequently, α-SiAlON powder and/or β-SiAlON powder may be treated at atemperature of 1800 to 2000° C. and a pressure of 40 to 60 GPa, tothereby cause phase transformation of a part thereof to cubic SiAlON,and accordingly obtain c-SiAlON powder including cubic SiAlON. Forexample, in the case where a shock compression process is used for thetreatment for causing the phase transformation, a shock pressure ofapproximately 40 GPa and a temperature of 1800 to 2000° C. may be usedto obtain different-type hard phase powder in which cubic SiAlON andα-SiAlON and/or β-SiAlON are mixed. At this time, the shock pressure andthe temperature may be changed to control the ratio of the cubic SiAlONto the different-type hard phase grains.

Step of Mixing Different-Type Hard Phase Powder, Cubic Boron NitridePowder, and Binder Powder

To the different-type hard phase powder prepared in the above-describedway and the cubic boron nitride powder with an average grain size of 0.1to 3 μm, powder of a binder, which is at least one kind of binderselected from the group consisting of at least one kind of element outof titanium (Ti), zirconium (Zr), aluminum (Al), nickel (Ni), and cobalt(Co), nitrides, carbides, oxides, carbonitrides, and borides of theelements, and solid solutions thereof, is added and mixed. As thisbinder powder, powder of a metal element such as Al, Ni, Co having anaverage grain size of 0.01 to 1 μm, powder of an intermetallic compoundsuch as TiAl having an average grain size of 0.1 to 20 μm, or powder ofa compound such as TiN, ZrN, TiCN, TiAlN, Ti₂AlN, TiB₂, AlB₂ having anaverage grain size of 0.05 to 2 μm, for example, is preferably used.Preferably, 10 to 40 vol % of the binder powder is added, relative tothe total amount of the different-type hard phase powder, the cubicboron nitride powder, and the binder powder. If the amount of the addedbinder powder is less than 10 vol %, the fracture toughness of thesintered body is lower, which may result in lower fracture resistance.If the amount of the added binder powder is more than 40 vol %, thehardness of the sintered body is lower, which may result in lower wearresistance.

For mixing the powder, balls made of silicon nitride or alumina ofapproximately ϕ3 to 10 mm may be used as media to perform ball-millmixing for a short time of within 12 hours in a solvent such as ethanol,or perform mixing by means of a medialess mixing apparatus such asultrasonic homogenizer or wet jet mill, to thereby obtain a slurrymixture in which the different-type hard phase powder, the cubic boronnitride powder, and the binder powder are uniformly dispersed.

The slurry mixture thus obtained is air-dried, or dried with a spraydryer or slurry dryer, or the like, to thereby obtain a powder mixture.

Sintering Step

After the powder mixture is shaped by means of a hydraulic press or thelike, the shaped powder mixture is sintered by means of a high-pressuregenerator such as belt-type ultrahigh pressure press machine, under apressure of 3 to 7 GPa and at a temperature of 1200 to 1800° C. Prior tosintering, the shaped powder mixture may undergo preliminary sinteringto be compacted to a certain extent, which may then be sintered.Moreover, an SPS (spark plasma sintering) apparatus may be used tosinter the powder mixture under a pressure of 30 to 200 MPa and at atemperature kept at 1200 to 1600° C.

Second Embodiment: Cutting Tool

A cutting tool in another embodiment of the present invention is acutting tool including the sintered body in the above-described firstembodiment. The cutting tool in the present embodiment thus includes thesintered body in the first embodiment, and therefore exhibits both highwear resistance and high fracture resistance when cutting a nickel-basedheat-resistant alloy formed of crystal grains with a fine grain sizerepresented by a grain size number of more than 5 defined under ASTMstandard E112-13. The cutting tool in the present embodiment maysuitably be used for cutting a difficult-to-work material such asheat-resistant alloy at a high speed. The nickel-based heat-resistantalloy used for parts of an aircraft or automobile engine is adifficult-to-work material which exhibits a high cutting resistance dueto its great high-temperature strength, and which is therefore likely tocause wear and/or fracture of the cutting tool. However, the cuttingtool in the present embodiment exhibits excellent wear resistance andfracture resistance even when cutting the nickel-based heat-resistantalloy. In particular, when cutting Inconel® 718 which is used for partsof an aircraft engine, the cutting tool used at a cutting speed of 100m/min or more exhibits an excellent tool life.

EXAMPLES Example 1

As the different-type hard phase grains, β-silicon nitride powder (SN-F1manufactured by Denka Company Limited, with an average grain size of 2μm), β-SiAlON powder (Z-2 manufactured by Zibo Hengshi TechnologyDevelopment Co., Ltd., with an average grain size of 2 μm), andα-alumina powder (TM-D manufactured by Taimei Chemicals Co., Ltd., withan average grain size of 0.1 μm) were used. Additionally c-SiAlON powdersynthesized in the following way was used as the different-type hardphase grains.

As to preparation of the c-SiAlON powder, a mixture obtained by mixing500 g of β-SiAlON powder and 9500 g of copper powder functioning as heatsink was placed in a steel pipe, and thereafter shock-compressed with anexplosive of an amount which was set so that the temperature was 1900°C. and the shock pressure was 40 GPa, to thereby synthesize the c-SiAlONpowder including cubic SiAlON. The powder mixture in the steel pipeafter being shock-compressed was removed, and acid-washed to remove thecopper powder. In this way, the synthesized powder was obtained. AnX-ray diffractometer (X'pert Powder manufactured by PANalytical, Cu-Kαray, 2θ-θ method, voltage×current: 45 kV×40 A, range of measurement:2θ=10 to 80°, scan step: 0.03°, scan rate: one step/sec) was used toanalyze the synthesized powder. Then, cubic SiAlON (JCPDS card:01-074-3494) and β-SiAlON (JCPDS card: 01-077-0755) were identified.From an X-ray diffraction pattern of the synthesized powder, the peakintensity Ic₍₃₁₁₎ of (311) plane which was a main peak of the cubicSiAlON, and the peak intensity Iβ₍₂₀₀₎ of (200) plane which was a mainpeak of β-SiAlON, were determined. The peak intensity ratio Rc of thecubic SiAlON calculated from the above-indicated formula (I) was 95%.

For each of Samples No. 1-1 to No. 1-14, TiN powder (TiN-01 manufacturedby Japan New Metals Co., Ltd., with an average grain size of 1 μm) wasadded as a binder at the ratio indicated in Table 1, to a total amountof 30 g of the different-type hard phase powder and the cubic boronnitride powder (SBN-F G1-3 manufactured by Showa Denko K.K., with anaverage grain size of 2 μm). For Samples No. 1-3 and No. 1-4, both theβ-SiAlON powder and the c-SiAlON powder were added at different ratiosof the c-SiAlON grains in the SiAlON included in the sintered body. Foreach of Samples No. 1-1 to No. 1-16, the amount (vol %) of the addedbinder powder was equal to the volume ratio (vol %) of the binder to thetotal amount of the different-type hard phase grains, the cubic boronnitride grains, and the binder in the sintered body shown in Table 1.Moreover, for each of Samples No. 1-1 to No. 1-14, the different-typehard phase powder and the cubic boron nitride powder were blended sothat their volume ratio was equal to the ratio V_(BN)/V_(H) of thevolume V_(BN) of the cubic boron nitride grains to the volume V_(H) ofthe different-type hard phase grains in the sintered body shown inTable 1. The powder, after the blending, of each of Samples No. 1-1 toNo. 1-14 was placed in a pot made of polystyrene with a capacity of 150ml, together with 60 ml of ethanol and 200 g of silicon nitride balls ofϕ6 mm, and subjected to ball mill mixing for 12 hours. A slurry mixturewas thus prepared. The slurry mixture removed from the pot wasair-dried, and thereafter passed through a sieve with a mesh opening of45 μm. Powder to be sintered was thus prepared.

Moreover, Sample No. 1-15 was prepared by mixing only the cubic boronnitride powder and TiN powder as a binder, without adding thedifferent-type hard phase powder. For Sample No. 1-15, fine cubic boronnitride powder (SBN-F G-1 manufactured by Showa Denko KK., with anaverage grain size of 1 μm) was used as the cubic boron nitride powder.

Moreover, Sample No. 1-16 was prepared by mixing only the cubic boronnitride powder and Co powder (HMP manufactured by Umicore) as a binder,without adding the different-type hard phase powder. For Sample No.1-16, the same cubic boron nitride powder as that of No. 1-1 to No. 1-14was used.

The powder to be sintered of each of Samples No. 1-1 to No. 1-16prepared in the above-described manner was vacuum-packed in a refractorymetal capsule with a diameter of ϕ20 mm, and thereafter electricallyheated to a temperature of 1500° C. while being pressurized to apressure of 5 GPa by means of a belt-type ultrahigh pressure press, tothereby prepare a sintered body.

The surface of the sintered body was surface-ground by means of a #400diamond abrasive, and thereafter X-ray diffraction of the ground surfacewas performed by means of the aforementioned X-ray diffractometer. Froman obtained diffraction pattern, the peak intensity Ic₍₃₁₁₎ of (311)plane of the cubic SiAlON and the peak intensity Iβ₍₂₀₀₎ of (200) planeof the β-SiAlON were determined, and the peak intensity ratio Rc of thecubic SiAlON (Rc=Ic₍₃₁₁₎/(Ic₍₃₁₁₎+Iβ₍₂₀₀₎)×100) was calculated. As aresult of this, there was substantially no change from the value of thepeak intensity ratio Rc of the cubic SiAlON before sintering, to thevalue thereof after sintering, for any of the sintered bodies of SamplesNo. 1-3 to No. 1-9 in which the cubic SiAlON was added.

After a cross section of the sintered body was mirror-polished with aCP, an FE-SEM (field emission scanning electron microscope) was used toobserve the structure of the sintered body, and an EDX (energydispersive X-ray spectroscopy) system integrated with the FE-SEM wasused to examine constituent elements of the crystal grains in thestructure of the sintered body and thereby identify the different-typehard phase grains, the cubic boron nitride grains, and the binder in animage of the SEM. The SEM image was image-processed with WinROOFmanufactured by Mitani Corporation, to thereby determine the area ratiobetween the different-type hard phase grains, the cubic boron nitridegrains, and the binder, and the area ratio was regarded as the volumeratio. In this way, the volume ratio between the different-type hardphase grains, the cubic boron nitride grains, and the binder included inthe sintered body was determined. As a result of this, in any ofrespective sintered bodies of Samples No. 1-1 to No. 1-14, the ratioV_(BN)/V_(H) of the volume V_(BN) of the cubic boron nitride grains tothe volume V_(H) of the different-type hard phase grains in the sinteredbody was substantially identical to the ratio of the volume of the cubicboron nitride powder to the volume of the different-type hard phasepowder as blended. Moreover, in any of respective sintered bodies ofSamples No. 1-1 to No. 1-16, the content of the hard-phase grains in thesintered body (the total content of the different-type hard phase grainsand the cubic boron nitride grains) (vol %) was substantially identicalto the ratio of the hard-phase grains as blended (the total ratio of thedifferent-type hard phase powder and the cubic boron nitride powder asblended) (vol %).

From the sintered body, a sample with a diameter of 18 mm and athickness of 1 mm was cut as a sample to be used for measuring thethermal conductivity, and a laser-flash-method thermal constantmeasuring apparatus (LFA447 manufactured by NETZSCH) was used to measurethe specific heat and the thermal diffusivity. The thermal conductivitywas calculated by multiplying the thermal diffusivity by the specificheat and the density of the sintered body. The results are shown inTable 1.

From the sintered body, a sample to be used for measuring the hardnesswas cut and embedded in a Bakelite resin. After this, the sample waspolished for 30 minutes with diamond abrasive grains of 9 μm and for 30minutes with diamond abrasive grains of 3 μm. A Vickers hardness tester(HV-112 manufactured by Akashi) was used to press a diamond indenterinto a polished surface of the sample with a load of 10 kgf. From theindentation formed by the pressing of the diamond indenter, the Vickershardness H_(V10) was determined. Further, the length of a crackextending from the indentation was measured. Further, the length of acrack extending from the indentation was measured and, based on the IFmethod under JIS R 1607: 2010 (Testing methods for fracture toughness offine ceramics at room temperature), the fracture toughness value wasdetermined. The results are shown in Table 1.

Next, the sintered body was processed into the shape of the brazedinsert of DNGA150412 (ISO model number), and the tool life of the brazedinsert was evaluated by using the insert for turning of Inconel® 718(manufactured by Daido-Special Metals Ltd.) with crystal grains having afine grain size represented by a grain size number of 9 defined byAmerican Society for Testing and Materials (ASTM) standard E112-13.Under the following conditions, an external cylindrical turning test wasconducted. A cutting length at which one of the flank face wear and theflank face fracture of the tool cutting edge reached 0.2 mm before theother was determined, and the determined cutting length was regarded asa tool life (km). The results are shown in Table 1. The life factorindicating whether the factor that caused the tool to reach the end ofthe tool life was wear or fracture is also shown in Table 1.

<Cutting Conditions>

The cutting conditions in the present Example are as follows.

-   -   workpiece: Inconel® 718 (manufactured by Daido-Special Metals        Ltd., solution heat-treated and age-hardened material, with a        Rockwell hardness HRC (a diamond cone with a tip radius of 0.2        mm and a tip angle of 120° was used to apply a load of 150 kgf)        corresponding to 44, and with a grain size represented by a        grain size number of 9 defined by ASTM standard E112-13)    -   tool shape: DNGA150412 (ISO model number)    -   cutting edge shape: chamfer angle −20°×width 0.1 mm    -   cutting speed: 200 m/min    -   depth of cut: 0.3 mm    -   feed rate: 0.2 mm/rev    -   wet condition (water soluble oil)

TABLE 1 Sample No. 1-1 1-2 1-3 1-4 1-5 1-6 1-7 1-8 different-type hardphase β- β- β- β- c- c- c- c- grains silicon SiAlON SiAlON SiAlON SiAlONSiAlON SiAlON SiAlON nitride c- c- SiAlON SiAlON content of hard phasegrains 80 80 80 80 80 80 80 80 (vol %) content of binder 20 20 20 20 2020 20 20 (vol %) ratio V_(BN)/V_(H) 3 3 3 3 0.8 1 3 6 peak intensityratio Rc (%) of — 0 15 20 90 90 90 90 cubic SiAlON thermal conductivity35 32 31 30 12 15 25 38 (W · m⁻¹ · K⁻¹) physical Vickers 23.7 24.1 24.525.2 22.3 22.9 31.2 35.3 properties hardness of sintered (GPa) bodyfracture 5.7 5.8 6.0 6.2 5.5 5.7 7.5 7.8 toughness (MPa · m^(1/2))cutting cutting 0.6 0.6 0.7 1.5 0.2 0.8 2.4 1.0 performance length (km)life factor wear wear wear wear wear wear wear fracture notes EX EX EXEX CE EX EX EX Sample No. 1-9 1-10 1-11 1-12 1-13 1-14 1-15 1-16different-type hard phase c- α- α- α- α- α- none none grains SiAlONalumina alumina alumina alumina alumina content of hard phase grains 8095 90 60 55 80 70 80 (vol %) content of binder 20 5 10 40 45 20 30 20(vol %) ratio V_(BN)/V_(H) 6.5 3 3 3 3 3 — — peak intensity ratio Rc (%)of 90 — — — — — — — cubic SiAlON thermal conductivity 43 35 30 18 16 2025 50 (W · m⁻¹ · K⁻¹) physical Vickers 33.0 32.5 30.3 22.3 21.5 24.829.5 38.0 properties hardness of sintered (GPa) body fracture 6.4 4.75.3 5.8 6.5 5.5 5.2 7.3 toughness (MPa · m^(1/2)) cutting cutting 0.20.3 0.6 0.8 0.4 0.8 0.4 0.2 performance length (km) life factor fracturefracture fracture wear wear wear fracture fracture notes CE EX EX EX EXEX EX CE EX: Example CE: Comparative Example

Referring to Table 1, the sintered bodies of Samples No. 1-5, No. 1-9,and No. 1-16 having a thermal conductivity of less than 15 W·m⁻¹·K⁻¹ ormore than 40 W·m⁻¹·K⁻¹ reached the end of the tool life when the cuttinglength reached 0.2 km. The sintered bodies of Samples No. 1-1 to No.1-4, No. 1-6 to No. 1-8, and No. 1-10 to No. 1-15 having a thermalconductivity of not less than 15 W·m⁻¹·K⁻¹ and not more than 40W·m⁻¹·K⁻¹ reached the end of the tool life when the cutting lengthreached 0.3 to 2.4 km, and the tool life of these sintered bodies wasconsiderably longer, namely 1.5 to 12 times as long as that of thesintered bodies of Sample No. 1-5, 1-9, or 1-16.

As to Sample No. 1-1, the different-type hard phase grains forming thesintered body were β-silicon nitride grains and the Vickers hardnessremained to be 23.7 GPa. As a result of this, this sample reached theend of the tool life due to wear when the cutting length reached 0.6 km.Sample No. 1-1 had a shorter life than Sample No. 1-4.

As to Sample No. 1-2, the different-type hard phase grains forming thesintered body were β-SiAlON grains and the Vickers hardness remained tobe 24.1 GPa. As a result of this, this sample reached the end of thetool life due to wear when the cutting length reached 0.6 km. Sample No.1-2 had a shorter life than Sample No. 1-4.

As to Sample No. 1-3, while the different-type hard phase grains formingthe sintered body included cubic SiAlON grains, the peak intensity ratioRc of the cubic SiAlON was an insufficient ratio of 15% and the Vickershardness remained to be 24.5 GPa. As a result of this, this samplereached the end of the tool life due to wear when the cutting lengthreached 0.7 km. Sample No. 1-3 had a shorter life than Sample No. 1-4.

As to Sample No. 1-5, because of a low ratio V_(BN)/V_(H) of 0.8 of thevolume V_(BN) of the cubic boron nitride grains to the volume V_(H) ofthe different-type hard phase grains forming the sintered body, thethermal conductivity was a low thermal conductivity of 12 W·m⁻¹·K⁻¹.This sample reached the end of the tool life due to wear when thecutting length reached 0.2 km.

As to Sample No. 1-9, because of a high ratio V_(BN)/V_(H) of 6.5 of thevolume V_(BN) of the cubic boron nitride grains to the volume V_(H) ofthe different-type hard phase grains forming the sintered body, thethermal conductivity was a high thermal conductivity of 43 W·m⁻¹·K⁻¹. Asa result of this, the temperature of the cutting edge of the tooldecreased during cutting, and thus the cutting resistance increased anda boundary damage of the cutting edge increased. Accordingly, thecutting edge of the tool fractured. Due to this, the sample reached theend of the tool life when the cutting length reached 0.2 km.

As to Sample No. 1-10, because of a high content of 95 vol % of the hardphase grains in the sintered body (the total content of thedifferent-type hard phase grains and the cubic boron nitride grains),the fracture toughness was 4.7 MPa·m^(1/2). As a result of this, thecutting edge of the tool fractured and thereby the sample reached theend of the tool life when the cutting length reached 0.3 km. Sample No.1-10 had a shorter life than Sample No. 1-11.

As to Sample No. 1-13, because of a low content of 55 vol % of the hardphase grains in the sintered body (the total content of thedifferent-type hard phase grains and the cubic boron nitride grains),the Vickers hardness remained to be 21.5 GPa. As a result of this, thesample reached the end of the tool life due to wear when the cuttinglength reached 0.4 km. Sample No. 1-13 had a shorter life than SampleNo. 1-12.

As to Sample No. 1-15, since fine cubic boron nitride grains were usedand TiN powder was used as a binder, the thermal conductivity was 25W·m⁻¹·K⁻¹ and the tool life was longer than that of Sample No. 1-16.However, since the sintered body did not include different-type hardphase grains, the toughness was low and this sample reached the end ofthe tool life due to fracture when the cutting length reached 0.4 km.

In contrast, as to Samples No. 1-4, No. 1-6 to No. 1-8, No. 1-12, andNo. 1-14 for which the peak intensity ratio Rc of cubic SiAlON in thedifferent-type hard phase grains forming the sintered body, the ratioV_(BN)/V_(H) of the volume V_(BN) of the cubic boron nitride grains tothe volume V_(H) of the different-type hard phase grains forming thesintered body, and/or the content of the hard phase grains in thesintered body (the total content of the different-type hard phase grainsand the cubic boron nitride grains) were controlled so that they were inrespective appropriate ranges, the well-balanced Vickers hardness andfracture toughness were obtained. As a result of this, the cuttinglength at which the sample reached the end of the tool life due to wearor fracture could be extended to 0.8 km or more.

In particular, as to Sample No. 1-7 including cubic boron nitridegrains, it was found that this sample having excellent Vickers hardnessand fracture toughness accordingly had a longer tool life than SamplesNo. 1-1 including silicon nitride grains and Sample No. 1-14 includingalumina grains.

As for Sample No. 1-16 including no different-type hard phase grains,the thermal conductivity was 50 W·m⁻¹·K⁻¹. As a result of this, thetemperature of the cutting edge of the tool decreased during cutting andthus the cutting resistance increased and a boundary damage of thecutting edge increased. Accordingly, the cutting edge of the toolfractured. Due to this, the sample reached the end of the tool life whenthe cutting length reached 0.2 km.

Example 2

C-SiAlON powder which was synthesized through shock compression in asimilar manner to Example 1 and in which cubic SiAlON had a peakintensity ratio Rc of 95% was used as different-type hard phase powderto be used for preparing respective sintered bodies of Samples No. 2-1to No. 2-10. The same cubic boron nitride powder (SBN-F G1-3manufactured by Showa Denko K.K.) as that used for Samples No. 1-1 toNo. 1-14 in Example 1 was used as cubic boron nitride powder of SamplesNo. 2-1 to No. 2-10.

For each of Samples No. 2-1 to No. 2-10, the binder powder shown inTable 2 was added to 30 g in total of the different-type hard phasepowder and the cubic boron nitride powder, so that the content of thebinder powder to the total amount of the different-type hard phasepowder, the cubic boron nitride powder and the binder powder was 20 vol%. At this time, for each of Samples No. 2-1 to No. 2-10, thedifferent-type hard phase powder and the cubic boron nitride powder wereblended so that the volume ratio therebetween was equal to the ratioV_(BN)/V_(H) of 3 of the volume V_(BN) of the cubic boron nitride grainsto the volume V_(H) of the different-type hard phase grains in thesintered body. Moreover, as the binder powder, TiCN powder (TiN-TiC50/50 manufactured by Japan New Metals Co., Ltd., with an average grainsize of 1 μm), TiN powder (TiN-01 manufactured by Japan New Metals Co.,Ltd., with an average grain size of 1 μm), TiAl powder (TiAlmanufactured by KCM Corporation), Al powder (300F manufactured byMinalco Ltd.), Co powder (HMP manufactured by Umicore), ZrN powder(ZrN-1 manufactured by Japan New Metals Co., Ltd.), and Ti₂AlN powder(with an average grain size of 1 μm) were used. For Samples No. 2-8 toNo. 2-10, the ceramic component TiN, TiCN, Ti₂AlN and the metalcomponent Co or Al were blended at a ratio by mass of 2 (ceramiccomponent) to 1 (metal component).

For each of Samples No. 2-1 to No. 2-10, the powder obtained after theblending was placed in a pot made of polystyrene with a capacity of 150ml, together with 60 ml of ethanol and 200 g of silicon nitride balls ofϕ6 mm, and subjected to ball mill mixing for 12 hours. A slurry was thusprepared. The slurry removed from the pot was air-dried, and thereafterpassed through a sieve with a mesh opening of 45 μm. Powder to besintered was thus prepared.

The powder to be sintered of each of Samples No. 2-1 to No. 2-10prepared in the above-described manner was vacuum-packed in a refractorymetal capsule with a diameter of ϕ20 mm, and thereafter electricallyheated to a temperature of 1500° C. while being pressurized to apressure of 5 GPa by means of a belt-type ultrahigh pressure press, tothereby prepare a sintered body.

The surface of the sintered body was surface-ground by means of a #400diamond abrasive, and thereafter X-ray diffraction of the ground surfacewas performed by means of an X-ray diffractometer. From an obtaineddiffraction pattern, the peak intensity Ic₍₃₁₁₎ of (311) plane of thecubic SiAlON and the peak intensity Iβ₍₂₀₀₎ of (200) plane of theβ-SiAlON were determined, and the peak intensity ratio Rc(Ic₍₃₁₁₎/(Ic₍₃₁₁₎+Iβ₍₂₀₀₎)×100) was calculated. The results are shown inTable 2.

After a cross section of the sintered body was mirror-polished with aCP, the volume ratio between the different-type hard phase grains, thecubic boron nitride grains, and the binder included in the sintered bodywas determined, in a similar manner to Example 1. As a result of this,in any of the sintered bodies of Samples No. 2-1 to No. 2-10, the ratioV_(BN)/V_(H) of the volume V_(BN) of the cubic boron nitride grains tothe volume V_(H) of the different-type hard phase grains in the sinteredbody was substantially 3. Moreover, the content of the hard phase grainsin the sintered body (the total content of the different-type hard phasegrains and the cubic boron nitride grains) was approximately 80 vol %.

From the sintered body, a sample with a diameter of 18 mm and athickness of 1 mm was cut as a sample to be used for measuring thethermal conductivity, and the thermal conductivity of respectivesintered bodies of Samples No. 2-1 to No. 2-10 was calculated in asimilar manner to Example 1. The results are shown in Table 2.

From the sintered body, a sample to be used for measuring the hardnesswas cut, and the Vickers hardness H_(V10) and the fracture toughnessvalue of respective sintered bodies of Samples No. 2-1 to No. 2-10 weredetermined in a similar manner to Example 1. The results are shown inTable 2.

Next, the sintered body was processed into the shape of the brazedinsert of DNGA150412 (ISO model number), and the tool life of the brazedinsert was evaluated by using the insert for turning of Hastelloy® Xwith crystal grains having a fine grain size represented by a grain sizenumber of 6 defined by ASTM standard E112-13. Under the followingconditions, an external cylindrical turning test was conducted. Acutting length at which one of the flank face wear and the flank facefracture of the tool cutting edge reached 0.2 mm before the other wasdetermined, and the determined cutting length was regarded as a toollife (km). The results are shown in Table 2. The life factor indicatingwhether the factor that caused the tool to reach the end of the toollife was wear or fracture is also shown in Table 2.

<Cutting Conditions>

The cutting conditions in the present Example are as follows.

-   -   workpiece: Hastelloy® X (manufactured by Haynes International,        Inc., solid solution heat-treated material, with a Brinell        hardness HB corresponding to 170, and with a grain size        represented by a grain size number of 6 defined by ASTM standard        E112-13)    -   tool shape: DNGA150412 (ISO model number)    -   cutting edge shape: chamfer angle −20°×width 0.1 mm    -   cutting speed: 200 m/min    -   depth of cut: 0.2 mm    -   feed rate: 0.1 mm/rev    -   wet condition (water soluble oil)

TABLE 2 Sample No. 2-1 2-2 2-3 2-4 2-5 2-6 2-7 2-8 2-9 2-10 binder TiNTiCN TiAl Al Co ZrN Ti₂AlN TiN, TiCN, TiN, Co Al Al peak intensity ratioRc 90 84 73 52 61 80 74 71 70 66 (%) of cubic SiAlON thermalconductivity 25 27 22 38 36 18 30 28 32 30 (W · m⁻¹ · K⁻¹) physicalVickers 31.2 32.3 30.5 28.5 27.5 30.8 30.3 29.8 31.1 30.2 propertieshardness of sintered (GPa) body fracture 7.5 7.0 7.9 8.3 8.4 6.7 8.0 7.88.0 8.3 toughness (MPa · m^(1/2)) cutting cutting 0.7 0.7 0.8 0.6 0.60.7 0.9 1.0 1.5 1.5 performance length (km) life factor fracturefracture wear wear wear fracture wear fracture wear wear notes EX EX EXEX EX EX EX EX EX EX EX: Example

Referring to Table 2, the sintered bodies of Samples No. 2-1 to No. 2-10with a thermal conductivity of not less than 15 W·m⁻¹·K⁻¹ and not morethan 40 W·m⁻¹·K⁻¹ had a long tool life corresponding to a cutting lengthof 0.6 to 1.5 km.

As to Samples No. 2-4 and No. 2-5 in which the metal component was usedas the binder, the sintered body had high fracture toughness. However,the sintered body had relatively high thermal conductivity. Therefore,the sintered body had a tool life corresponding to a cutting length of0.6 km due to fracture.

In contrast, as to Samples No. 2-1 to No. 2-3, No. 2-6, and No. 2-7 inwhich the binder was the ceramic or intermetallic binder, thewell-balanced thermal conductivity and Vickers hardness could beobtained. As a result, the cutting length at which the end of the toollife was reached due to wear or fracture could be extended to 0.7 km ormore.

As for Samples No. 2-8 to No. 2-10 in which both the ceramic componentand the metal component were used as the binder, the sintered bodiesexhibited excellent Vickers hardness and fracture toughness. Therefore,the cutting length at which the end of the tool life was reached was 1.0km or more.

It should be construed that the embodiments and examples disclosedherein are given by way of illustration in all respects, not by way oflimitation. It is intended that the scope of the present invention isdefined by claims, not by the description above, and encompasses allmodifications and variations equivalent in meaning and scope to theclaims.

INDUSTRIAL APPLICABILITY

As seen from the foregoing, the sintered body including cubic boronnitride grains include both the cubic boron nitride grains havingexcellent hardness and toughness and the ceramic grains having lowthermal conductivity, to thereby provide an advantage that the sinteredbody is excellent in wear resistance when used for cutting adifficult-to-cut material such as nickel-based heat-resistant alloywhich has high cutting resistance and which does not easily soften. Inaddition, the sintered body provides a tool material improving thefracture resistance of the cutting edge of the cutting tool. While theeffects produced when cutting Inconel® are disclosed herein inconnection with the Examples, the sintered body exhibits excellent wearresistance and fracture resistance when used for cutting adifficult-to-cut material such as titanium (Ti) other than theheat-resistant alloy such as Inconel®, and is particularly applicable tohigh-speed cutting.

1. A method for manufacturing a cut material, the method comprising:preparing a nickel-based heat-resistant alloy formed of crystal grainshaving a fine grain size represented by a grain size number of more than5 defined by American Society for Testing and Materials standard E112-13cutting the nickel-based heat-resistant alloy by using a sintered bodycomprising cubic boron nitride grains as hard phase grains and having athermal conductivity of not less than 15 W·m⁻¹·K⁻¹ and not more than 40W·m⁻¹·K⁻¹.
 2. The method for manufacturing the cut material according toclaim 1, wherein the sintered body further comprises: a binder; anddifferent-type hard phase grains including at least one selected fromthe group consisting of silicon nitride, SiAlON, and alumina, as thehard phase grains other than the cubic boron nitride grains.
 3. Themethod for manufacturing the cut material according to claim 2, whereina ratio V_(BN)/V_(H) of a volume V_(BN) of the cubic boron nitridegrains to a volume V_(H) of the different-type hard phase grains is notless than 1 and not more than
 6. 4. The method for manufacturing the cutmaterial according to claim 2, wherein the SiAlON includes cubic SiAlON.5. The method for manufacturing the cut material according to claim 4,wherein the SiAlON further includes at least one of α-SiAlON andβ-SiAlON, and a peak intensity ratio Rc of an intensity at an X-raydiffraction main peak of the cubic SiAlON to a sum of respectiveintensities at respective X-ray diffraction main peaks of the α-SiAlON,the β-SiAlON, and the cubic SiAlON is not less than 20%.
 6. The methodfor manufacturing the cut material according to claim 2, wherein thebinder includes at least one kind of binder selected from the groupconsisting of at least one kind of element out of titanium, zirconium,aluminum, nickel, and cobalt, nitrides, carbides, oxides, carbonitrides,and borides of the elements, and solid solutions thereof.
 7. The methodfor manufacturing the cut material according to claim 1, wherein acontent of the hard phase grains in the sintered body is not less than60 vol % and not more than 90 vol %.
 8. The method for manufacturing thecut material according to claim 1, wherein the sintered body has aVickers hardness of not less than 22 GPa.
 9. The method formanufacturing the cut material according to claim 1, wherein thenickel-based heat-resistant alloy includes nickel of not less than 50mass % and not more than 55 mass %, chromium of not less than 17 mass %and not more than 21 mass %, niobium of not less than 4.75 mass % andnot more than 5.50 mass %, molybdenum of not less than 2.80 mass % andnot more than 3.30 mass %, and iron of not less than 12 mass % and notmore than 24 mass %.
 10. The method for manufacturing the cut materialaccording to claim 1, wherein the cutting of the nickel-basedheat-resistant alloy is performed by using a cutting tool comprising thesintered body.